The present invention relates to a translucent screen comprising a lens system, in particular a screen with a Fresnel lens for use as or in connection with a projection screen, and preferably for use in a rear projection screen, and a projection screen with such Fresnel lens. The present invention also relates to methods of manufacturing a translucent screen according to the invention.
Projection screens with Fresnel lenses are used in various apparatuses for generating an image that is visible to the viewer; eg rear projection screens are used in connection with the display of radar images, in flight simulators, control rooms, television sets, video monitors, traffic control lights, microfilm readers, video-games and for the showing of films. In such apparatuses an image source arranged behind the screen projects light forwards along a projection axis towards the screen with a view to forming an image on the front of the screen that is visible to the viewer. Typically the screens are rectangular and may have many different dimensions, eg a screen for a microfilm reader will have a diagonal of about 38 cm (15 inches), whereas a screen for a control room or showing film can have a diagonal as large as about 450 cm (180 inches) or more.
A projection screen consists of two functional elements, partly a first element for converting the diverging light beams from the image source to parallel beams, partly a diffusion element that spreads the light from the first element in order to thereby make it visible for a viewer. In practice the first element consists of a substantially plane Fresnel lens structure and the second element of a plane plate with light-diffusing properties.
In principle such screen can be constructed in two ways, partly with a single plane sheet element that is, on the side facing towards the image source, provided with a Fresnel lens, and on the other side with a light-diffusing coating or structure, partly with two plane sheet elements arranged parallel in front of each other, wherein the sheet element most proximate to the image source is provided with a Fresnel lens on that side of the sheet that faces away from the image source, and wherein the sheet element that faces towards the viewer is provided with a light-diffusing coating or structure.
The drawback of the first principle is that a Fresnel lens that faces directly towards the light source has a relatively large transmission loss, typically of about 15 to 20 percent. This is due to the fact that a part of the light hits the step faces of the Fresnel lens and are therefor spread in an undesired direction; this phenomenon increases towards the periphery of the lens where the height of the step faces is increased which means that the loss of light is most comprehensive corresponding to the periphery of the screen. An advantage of this configuration is a more simple construction.
In the other principle where the Fresnel lens is arranged on that side of the sheet element that faces away from the image source, all light that moves into the plate hits the xe2x80x98activexe2x80x99 Fresnel facets where it is deflected to the: above-described parallel batch of beams. Albeit in principle this construction entails an increased efficiency of transmission, the separate light-diffusing plate, however, will cause a loss of transmission when the light is to pass two more border faces and therefore this type of screen has a transmission effect increase of no more than five to ten percent in all, a value that must in turn take into consideration the more complex construction thereof.
The drawback of both principles is the formation of image disturbances, such as rainbows or double- or multiple-image formation, also designated ghost images. Such phenomena are due to reflections that originate in the step faces at the lens that faces backwards and from the rear face of the Fresnel facets of the forwardly oriented lens, respectively. It applies to both principles that the disturbances are most expressed corresponding to the periphery of the lens where the facets are most steep and have the highest step faces. It also follows from this that the most comprehensive problems occur with lenses with short focal lengths since they are provided with the steepest facets.
The problem with internal reflections is well documented and various attempts have been made to counter them.
For instance, WO 99/53376 describes a projection-screen assembly comprising a layer with a Fresnel lens structure on the one side, wherein this layer consists of a matrix with refractive particles distributed therein. The particles are distributed throughout the entire thickness of the plate, and they serve on the one hand as image-generating diffusion means, and on the other hand to suppress disturbances created within the Fresnel construction elements as such, as also described above. This means that the same type of diffusion means is used for both functionalities, and likewise the positioning and properties of the image-generating parts are predetermined for the total screen structure.
The Fresnel layer is configured as an actual, self-supporting sheet that can be mounted either alone or in combination with other layers. Such plate will have a typical thickness of two to three mm. In case other layers are used the various layers can either be combined by gluing or mechanically. WO 99/53376 teaches various methods of manufacturing the Fresnel layer, eg extrusion of a plate with subsequent embossment of the Fresnel structure.
Also, EP-A-0 732 615 describes how a light-diffusing agent can be contained in a Fresnel lens that can, in turn, constitute one of several elements in a projection-screen assembly. This is also a case of the entire projection screen being constructed by combination of a number of layers, or screens.
U.S. Pat. No. 5,477,380 describes that reflections from the rear side of the facets can be attenuated by use of a lens basis containing a refractive diffusion material, but since, on the one hand, the refractive diffusion material is located in correspondence with that surface of the lens base plate that faces away from the lens facts, and on the other hand is very thick (in preferred embodiments the refractive material is distributed almost throughout the entire thickness of the base plate), a powerful diffusion of the incoming light beams will occur before they hit the back of the facets resulting in an unfocused and contrast-poor image. EP-A-0 859 270 discloses a corresponding solution in which the rear of the screen is coated with a relatively thick layer of a refractive diffusion material.
Japanese Patent Abstract 11 072 849 describes how the formation of rainbow phenomena can be reduced by use of a Fresnel lens, wherein the entire lens, ie both lens basis and lens facets, contain a refractive diffusion material. As mentioned above, this will lead to an unfocused as well as contrast-poor image. Also EP-A-0 859 270, U.S. Pat. No. 4,361,382 and Japanese Patent Abstract 10 293 361 teach screens wherein a refractive diffusion material is distributed corresponding to the entire thickness of the lens.
Accordingly it is an object of the invention to provide a screen comprising a surface with a number of lens facets that combine to form a lens system for paralleling diverging light beams (in particular a Fresnel lens structure) and that is suitable for use in or for acting as a projection screen, and wherein the problems with rainbows and double- or multiple-image formation has been reduced to a minimum while maintaining high definition and adequate contrast in image transmission.
It is a further object of the invention to provide an effective and simple method of manufacturing projection screens according to the invention.
The above and further objects of the invention that will appear from the description that follows of preferred embodiments of the invention are accomplished in that a translucent screen comprising a lens system according to the invention contains a refractive diffusion material distributed corresponding essentially to the lens facets as such, or corresponding to the lens facets as such and a layer immediately behind same. This principle has surprisingly been found to yield a much improved image transmission compared to the above-described solutions, wherein the light-diffusing agent is found either throughout the entire screen, the entire Fresnel screen or in that part of the screen that is most distant from the lens facets.
Typically, the translucent screen comprises a sheet element with a first surface and a second surface substantially parallel with the first surface, and wherein the first surface comprises a number of lens facts that combine to form a lens system for paralleling diverging light beams that enter into the sheet element from the first or the second surface. The second surface can be substantially planar or it can comprise a further lens system, eg a lenticular lens system.
According to a first, preferred embodiment the screen consists of a single xe2x80x98unitaryxe2x80x99 layer, ie the lens facets as well as the underlying lens basis comprises a common matrix. According to a second preferred embodiment the screen comprises a first matrix and a second matrix, wherein the lens facets comprise the first matrix and the underlying lens basis comprises the second matrix, the latter typically as a continuous layer in the form of a sheet. The terms xe2x80x98first and second matrixxe2x80x99 are used to designate that each of the two matrices has at least one property that is not shared.
When it is described in the present application that a refractive diffusion material or a matrix material is distributed corresponding essentially to the lens facets as such, this means that also that part of the lens basis that is most proximate to the lens facets can contain the refractive material in a layer that is thin compared to the thickness of lens basis, ie less than 10 percent of the thickness of the base plate. This is typically due to the fact that often it is not technically possible to distribute the diffusion material only in the lens facts, but xe2x80x98bridgesxe2x80x99 of diffusion material will almost always occur between the individual lens facets. Since a base plate has a typical thickness of 2-3 mm, this means that the bridges can have a thickness of about 0.2 mm. However, it will often be possible to reduce the thickness considerably, eg as to as little as 0.005 mm when lens basis in the form of a solid plate is pressed towards a fluid matrix distributed over the lens facets.
The screens corresponding to the present invention can be used in combination with other screens and therefore the first and second surfaces of the screen need not be free, but can be mounted on or in connection with other screen elements.
According to a second aspect of the invention various methods are provided for effective and simple manufacture of screens according to the invention.